It is troublesome to constantly maintain ordinary house-hold knives in a sharpened condition due to the variations in actual time of usage of the knife. The user does not always remember when the blade should be sharpened until the knife is so dull that it will not cut. Often, it is not convenient to hone or otherwise sharpen the knife blade before each use and, over time, the cutting edge becomes dull, thus rendering the knife difficult to use. Furthermore, the extent of resharpening necessary depends directly upon the dullness of the knife with the duller knives requiring a considerable amount of metal to be removed to produce a precise, sharp cutting edge.
A number of prior art patents have recognized this deficiency with such ordinary knives, and provide a housing or scabbard for receiving, protecting and sharpening the cutting edge of a knife blade as the blade is inserted into or withdrawn from the scabbard. Typical patents in this area include U.S. Pat. Nos. 2,475,937, 2,744,320, 2,767,530, 3,676,961, 3,774,350, 3,861,246, 3,889,809, 4,041,651, 4,091,691 and 4,117,748. Each of these patents discloses a device which is designed to sharpen the knife blade upon its insertion into and/or withdrawal from the scabbard. Furthermore, certain of these patents, such as the '691 patent, for example, include a device which sharpens the knife both on insertion into and withdrawal from the scabbard. Each of these patents include means, such as a spring, a latch member or a resilient member, for biasing the knife blade toward the sharpening elements.
It has now been realized that these devices are deficient because they sharpen the blade upon each insertion and/or removal from the device, whether or not the blade requires sharpening. The sharpening action causes metal to be removed from the blade each time the knife is placed in and out of the scabbard. Most of the time, after a single use of the knife, it is not necessary to sharpen the blade. Hence, these devices provide an unnecessary sharpening which causes the eventual wearing away of the blade.
Another knife sharpener is disclosed in U.S. Pat. No. 4,494,339. This device is similar to those mentioned above in that a spring is used to press the knife blade edge against the sharpener; however, an adjustment knob is provided to vary the spring force and the extent of sharpening of the blade. This spring force may, if desired, be set to zero so as to render the sharpener ineffective.
The device described above is still deficient, however, in that the user of the knife must set the spring force each time the knife is inserted or withdrawn. As a practical matter, most users will set the adjustment at an intermediate position so that, in effect, the device performs the same as those discussed above.
The present invention provides a solution to this problem in a convenient cassette which both holds, protects, and, only when desired, simply and easily sharpens the blade of the knife to the desired extent. Use of this device avoids the continuous wearing of the blade, yet provides a sharpening device which is readily available for use at the desire of the user, and in which the amount of force used to sharpen the knife blade, if any, is imparted by the user.